The present invention relates to a press for forming composite allografts used in orthopedic surgery, and in particular to a press for forming composite acetabular allograft cups.
There is a need for methods of replacing or strengthening certain types of bone defects. One common example of such need applies in the case of hip replacement surgery. A hip joint is a ball and socket joint in which the ball is the femoral head and the socket is called the acetabulum (due to its supposed resemblance to a vinegar cruet). The cavity of the acetabulum is formed from three parts of the pelvic bone: above by the ilium, behind and below by the ischium, and internally by the os pubis. Patients who are otherwise candidates for hip replacement surgery may have acetabular defects. The acetabulum may for various reasons, including disease, trauma or prior surgery, contain defects such as missing or eroded portions of the acetabular wall. These defects must be corrected or compensated for if the surgery is to be successful.
In hip replacement surgery, a hip joint prosthesis, comprising a femoral component and an acetabular component, is employed to replace the femoral head and the acetabulum. The acetabular component may include a hemispherical metal cup or ring and a low-friction plastic liner of ultra-high molecular weight polyethylene. The procedure may also be done without the metal cup, using only the liner which is cemented in place.
One method of compensating for an acetabular defect is to repair the defect with a bone graft (either an allograft, typically harvested from a cadaver, or an autograft from the patient""s own bone tissue). Due to the significant weight-bearing role of the hip joint, the stability and strength of the bone graft is a major concern. Metallic support cups may be required to support the bone graft material as disclosed in MacCollum (U.S. Pat. No. 4,904,265). MacCollum discloses a support cup in the shape of a rigid metallic hemisphere with a flange to support the bone graft. The outer surface of the support cup is disclosed to be porous to support bone ingrowth. A bearing insert of low-friction material for receiving the ball of the femoral prosthesis is mounted within the support cup.
As an alternative to bone grafts, Grimes (U.S. Pat. No. 5,176,711) discloses an acetabular hip prosthesis in which the acetabular component of the prosthesis includes an augmentation piece to fill a rim or cavitary defect. Likewise, Collazo (U.S. Pat. No. 5,326,368) discloses a modular prosthetic acetabular cup to provide various cross sections as desired to fill acetabular defects.
Another method of remedying an acetabular defect is disclosed in xe2x80x9cBone Grafting in Total Hip Replacement for Acetabular Protrusionxe2x80x9d by McCollum, et al., Journal of Bone and Joint Surgery, Vol. 62-A, No. 7, 1065-1073 (October 1980). The McCollum article discloses the use of wafers of bone to fill a defect in the acetabular wall.
A slightly different technique is disclosed in xe2x80x9cBone Grafting in Total Hip Replacement for Acetabular Protrusionxe2x80x9d by Slooff, et al., Acta Orthop. Scand, 55, 593-596, (1984). While Slooff et al disclose the use of a bone graft to close an acetabular defect, Slooff et al. also disclose surrounding the graft with a wall of cancellous bone chips that are molded and impacted by using the socket trial prosthesis. (Cancellous bone has a spongy or lattice-like structure and may be derived from cadaverous bone tissue such as femoral heads.) Slooff et al. disclose a technique of repairing an acetabular defect in which cancellous bone chips are molded and impacted around a bone graft, but do not disclose the addition of cement to the impacted bone chips.
Gie, et al. in xe2x80x9cImpacted Cancellous Allografts and Cement for Revision Total Hip Arthroplasty,xe2x80x9d The Journal of Bone and Joint Surgery, Vol. 75-B, No. 1, 14-21 (January 1973) disclose the use of impacted cancellous allografts and cement for fixation of the femoral component in total hip arthroplasty. The technique disclosed by Gie et al. involves packing allograft bone chips into the femoral canal using the trial femoral component. The chips are repeatedly impacted after which cement is introduced and pressurized to force the cement into the graft. Pressure is maintained until the cement has sufficiently solidified. While Gie et al. disclose impacting cancellous bone chips into the femoral canal after which cement is added to the impacted bone chips and pressurized to force the cement into the graft, Gie et al. do not disclose the use of this technique in relation to the acetabulum. Neither Slooff et al. nor Gie et al. disclose the formation of a composite acetabular cup outside the body of the patient prior to surgery.
It is known to form human tissue into particular shapes to create desired natural tissue grafts. For example, U.S. Pat. No. 4,678,470 issued to Nashef et al. on Jul. 7, 1987 for xe2x80x9cBone-Grafting Materialxe2x80x9d discloses a bone-grafting material derived from allogenic or xenogenic bone that may be machined into a predetermined shape.
U.S. Pat. No. 5,329,846 issued to Bonutti on Jul. 19, 1994 for xe2x80x9cTissue Press and Systemxe2x80x9d discloses a press for shaping or compressing a piece of tissue by the movement of two members relative to each other. Various shapes of the two movable members may be selected so as to produce tissue in the desired shape. While the Bonutti invention is primarily directed to the compression and shaping of soft tissue, portions of the disclosure suggest the shaping of bone tissue with the addition of polymeric material (column 11, lines 11-13). Bonutti does not expressly disclose the formation of an acetabular cup using cancellous bone chips and cement. While Bonutti discloses the importance of monitoring and controlling the pressure applied to the compressed tissue, it is in the context of maintaining graft tissue in a living state to improve graft viability and tissue healing. In this context Bonutti discloses the use of pressure sensors and force-limiting means such as the mechanism found on torque wrenches. (See FIG. 6A.)
It is known to use pressure gauges, load limiting devices and the like in presses in the manufacturing environment. An example is U.S. Pat. No. 3,786,676 which discloses a compression testing machine having an in-line load cell.
The present invention is directed to a press for forming a composite acetabular allograft cup. The acetabular allograft press comprises a base and upper frame that are forced together to apply pressure to a two-piece mold in the shape of the required acetabular cup. Various sizes of molds may be employed for different patient requirements. Pressure is applied by a manually operated screw mechanism. A pressure gauge is used to indicate the correct degree of loading to the mold.
One potential problem with the use of a press is that the operator may have difficulty removing the allograft cup from the mold after it has set, since the bone cement used in forming the acetabular cup may adhere to the upper portion of the mold. To overcome this problem, the present invention includes breaking jaws that extend between the upper portion of the mold and the allograft cup when the mold is closed during acetabular cup formation. To separate the allograft cup from the upper portion of the mold, the operator merely presses down firmly on the handle mechanically linked to the breaking jaws. This forces the cup down and away from the upper portion of the cup, with a sufficient degree of force to break the bond between the acetabular cup and the upper portion of the mold.
The method of using the acetabular allograft press comprises the following steps:
(a) a quantity of cancellous bone chips is placed in the mold (cancellous bone chips are commercially available);
(b) pressure is applied to the bone chips to cause the chips to conform to the shape of the mold (it is important that the load applied to the bone chips is limited to avoid excessive crushing of the bone);
(c) the mold is opened and additional bone chips are added to fill any voids;
(d) a load is again placed on the bone chips to cause the newly added bone chips to conform to the shape of the mold;
(e) commercially available bone cement is added to the mold;
(f) the mold is again loaded and the load is maintained for a sufficient period of time for the cement to harden; and
(g) the allograft cup is separated from the upper portion of the mold using the breaking jaws so that the acetabular cup can be removed.
This process produces a synthetic composite acetabular cup in which the inner surface is smooth and comprised essentially of hardened bone cement material. The outer portion of the cup may have a limited proportion of cement extrusions but the major portion of the exterior of the cup shows exposed cancellous bone surface. The acetabular cup is therefore suited to provide a smooth, strong inner surface to receive an acetabular implant, while the outer surface is suited for encouraging bone growth from the acetabulum into the exposed bone of the acetabular cup. While this technique is disclosed with reference to the particular application of an acetabular cup, the same techniques offer advantages in other applications where an allograft having the described properties is desirable. The present invention should not, therefore, be seen as limited to one particular application.
In surgery, the acetabular cup is highly advantageous since it avoids the necessity of performing grafting to correct acetabular defects such as by the method disclosed in Gie et al. The acetabular cup made using the press of the present invention could be formed in advance of surgery. During surgery the acetabular cup is positioned in the acetabulum so as to fill the acetabular defect and fixed in place using screws. The remainder of the total hip replacement surgery would be carried out using well-known techniques. Using the surgical method of the present invention, however, a metal acetabular cup component is not required. A high-density plastic liner is affixed with bone cement directly to the synthetic composite acetabular cup to receive the femoral component. The use of the present invention is not limited to the acetabulum; the press could also be used to form synthetic allografts for other purposes in orthopedic surgery that would be apparent to one skilled in the art.
It is therefore an object of the present invention to provide for a press to form a synthetic composite allograft.
It is also an object of the present invention to provide for a press to form a synthetic composite acetabular cup for the repair of acetabular defects encountered in total hip replacement surgery.
It is a further object of the present invention to provide for a press to form synthetic composite allografts, and in particular a synthetic composite acetabular cup, which presents a smooth, strong inner surface of hardened cement material and an outer surface consisting essentially of compacted cancellous bone chips.
It is a further object of the present invention to provide for a press with a screw-type mechanism that includes breaking jaws so that the allograft cup may be easily separated from the mold and removed after the cup is formed.
Further objects and advantages of the present invention will become apparent from an examination of the detailed description of the preferred embodiments considered in conjunction with the appended drawings as described hereinafter.